Switches and bandpass filters (BPFs) are the basic blocks in many radio frequency (RF) sub-systems, such as the time division duplex (TDD) front-ends. Generally, they are cascaded, thus inter-stage mismatching may exist and thus cause performance degradation. Furthermore, the total loss is the sum of losses of the BPF and the switch, which is usually high. The isolation is often less than 30 dB, because of parasitic parameters of the switching transistors or diodes.
To reduce the loss and improve isolation, filtering single-pole single-throw (SPST) switches are proposed. The ON- and OFF-states are enabled by turning on or off the PIN diodes embedded in filter structures. For example, high-isolation filtering switches can be designed by employing the switchable connected-coupling lines and switchable delay lines, respectively. However, the signals pass through the PIN diodes in the ON-state, which introduces extra insertion loss and reduce the power handling capability. In additional, PIN diodes can be utilized to change the resonant frequencies of the resonators. In this way, high-order BPFs are needed to obtain high OFF-state isolation. Besides filtering SPST switches, the integration of single-pole double-throw (SPDT) switches and BPFs are also demanded and several methods have been proposed to facilitate the co-designs. For example, common resonators are shared by multiple sets of filters for size reduction and performance enhancement. Nevertheless, they suffer from limited power handling capability and thus are not suitable for high-power applications. The technologies used by the filtering switches mentioned above are difficult to realize high selectivity and narrow-band fractional bandwidths (FBWs), for example, less than 2%, due to Q-factor limitations, as most of the applications of the filtering switches are integrated on PCB or IC.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.